Surgically implantable knee prosthesis having medially shifted tibial surface

ABSTRACT

An implantable knee prosthesis includes a body having a substantially elliptical shape in plan and a pair of opposed faces. A peripheral edge of variable thickness extends between the faces and includes a first side, a second side opposite the first side, a first end and a second end opposite the first end. The thickness of the peripheral edge at the first side is greater than the thickness of the peripheral edge at the second side, the first end and the second end.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/044,756 filed Jan. 11, 2002, now abandoned, which is acontinuation-in-part of U.S. application Ser. No. 09/664,939, filed onSep. 19, 2000, now U.S. Pat. No. 6,558,421, which is a continuation ofU.S. application Ser. No. 09/297,943, filed May 10, 1999, now U.S. Pat.No. 6,206,927.

BACKGROUND

The present invention pertains to prosthetic devices. More particularly,the invention pertains to knee joint prostheses which may be surgicallyimplanted between the femoral condyle and tibial plateau of the kneejoint.

Articular cartilage and meniscal cartilage provide the mobile weightbearing surfaces of the knee joint. Damage to these surfaces isgenerally due to genetic predisposition, trauma, and/or aging. Theresult is usually the development of chondromalacia, thinning andsoftening of the articular cartilage, and degenerative tearing of themeniscal cartilage. Various methods of treatment are available to treatthese disease processes. Each option usually has specific indicationsand is accompanied by a list of benefits and deficiencies that may becompared to other options.

The healthy knee joint has a balanced amount of joint cartilage acrossthe four surfaces of this bi-compartmental joint (medial femoralcondyle, medial tibial plateau, lateral femoral condyle and lateraltibial plateau). In patients with osteoarthritis, degenerative processtypically leads to an asymmetric wear pattern that leaves onecompartment with significantly less articular cartilage covering thedistal portions (or weight bearing area) of the tibia and femur than theother compartment. Most commonly, the medial compartment of the kneejoint is affected more often than the lateral compartment.

As the disease progresses, large amounts of articular cartilage are wornaway. Due to the asymmetric nature of the erosion, the alignment of themechanical axis of rotation of the femur relative to the tibia becomestilted down towards the compartment which is suffering the majority ofthe erosion. The result is a Varus (bow-legged) deformity in the case ofa medial compartment disease predominance, or a Valgus (knock-kneed)deformity in the case of lateral compartment disease predominance.Factors such as excessive body weight, previous traumatic injury, kneeinstability, the absence of the meniscus and genetic predisposition, allaffect the rate of the disease.

The disease is usually defined in stages of Grade I through V, withGrade III revealing significant articular cartilage loss, Grade IVrevealing some eburnation of the subchondral bone, and Grade V detailingboth significant articular loss and bone loss.

It is important to understand that the disease manifests itself asperiodic to continuous pain that can be quite uncomfortable for thepatient. The cause of this pain is subject to many opinions but it isapparent that, as the joint compartment collapses, the collateralligament on the side of the predominant disease becomes increasinglyslack (like one side of a pair of loose suspenders), and the tibial andfemoral axes move, for example, from a Varus to a Valgus condition. Thisincreases the stress on the opposing collateral ligament (and cruciateligaments as well) and shifts the load bearing function of thisbi-compartmental joint increasingly towards the diseased side. Thisincreasing joint laxity is suspected of causing some of the pain onefeels. In addition, as the bearing loads are shifted, the body respondsto the increased loading on the diseased compartment with an increasedproduction of bony surface area (osteophytes) in an attempt to reducethe ever-increasing areal unit loading. All of this shifting of the kneecomponent geometry causes a misalignment of the mechanical axis of thejoint. The misalignment causes an increase in the rate of degenerativechange to the diseased joint surfaces causing an ever-increasing amountof cartilage debris to build up in the joint, further causing jointinflammation and subsequent pain.

Currently, there is a void in options used to treat the relatively youngpatient with moderate to severe chondromalacia involving mainly onecompartment of the knee. Current treatments include NSAIDs, cortisoneinjections, hyaluronic acid (HA) injections and arthroscopicdebridement. Some patients cannot tolerate or do not want the risk ofpotential side effects of NSAIDs. Repeated cortisone injections actuallyweaken articular cartilage after a long period of time. HA has shownpromising results but is only a short term solution for pain.Artliroscopic debridement alone frequently does not provide long lastingrelief of symptoms. Unfortunately, the lack of long term success ofthese treatments leads to more invasive treatment methods. Osteochondralallografts and microfracture techniques are indicated for smallcartilage defects that are typically the result of trauma. Theseprocedures are not suitable for addressing large areas of degeneration.In addition, osteochondral allografts can only be used to addressdefects on the femoral condyle. Tibial degeneration can not be addressedwith this technique. High tibial osteotomy (HTO) corrects the varusmalalignment between the tibia and femur but, because it is performedbelow the joint line, it does not fill the cartilage void or re-tensionthe medial collateral ligament (MCL). Removing bone and changing thejoint line does not complicate the conversion to total knee arthroscopy(TKA). However, an HTO does leave a hard sclerotic region of bone whichis difficult to penetrate making conversion to a total knee replacement(TKR) technically challenging. Unicompartmental and bicompartmentaltotal knee replacements resect significant amounts of bone and, ifperformed on younger patients, will likely require revision surgery asthey age. Revision total knee replacement surgery is usually extensiveand results in predictably diminished mechanical life expectancy.Therefore, it is best to delay this type of bone resecting surgery aslong as possible.

The only true solution is to rebuild the defective joint by “filling”the joint space with more articular bearing material through a completeresurfacing of the existing femoral condyle and tibial plateau. Byreplacing the original cartilage to its pre-diseased depth, the jointmechanical axis alignment is restored to its original condition.Unfortunately, these natural articular materials and surgical technologyrequired to accomplish this replacement task do not yet exist.

Currently, replacement of the existing surfaces, with materials otherthan articular cartilage, is only possible with a total or uni-condylarknee replacement, and these procedures require removal of significantamounts of the underlying bone structure.

The alternative method is to fill the joint space with a spacer thatreplaces the missing articular materials. This spacer should alsoprovide an anatomically correct bearing surface for both the tibial andfemoral surface (U.S. Pat. No. 6,206,927).

Attaching a new bearing surface to the femoral condyle is technicallychallenging and was first attempted, with limited success, over 40 yearsago with the MGH (Massachusetts General Hospital) knee. Like a dentalcrown, it covered both femoral condyles with Vitallium (CoCr) and wouldbear against the existing tibial plateau.

Tibial covering devices such as the McKeever, Macintosh and Townleytibial tray, maintained the existing femoral surface as the bearingsurface, but like the MGH knee, all required significant bone resection,thus making them less than ideal solutions as well.

These devices also made no particular attempt to match the patient'sspecific femoral or tibial geometry thus reducing their chances foroptimal success. Because these devices were made of CoCr, which hasdifferent visco-elastic and wear properties from the natural articularmaterials, any surface geometry which did not closely match the bearingsurface of the tibia or femur, could cause premature wear of theremaining cartilage due to asymmetric loading.

Newer materials technologies in development include filling the jointspace by injecting polyurethane (U.S. Pat. No. 5,795,353) into the jointand anchoring it with holes drilled into the tibial plateau. Othersinclude a series of polymeric materials such as PVA Hydrogels in atitanium mesh as described by Chang et al, “Historical Comparison ofTibial Articular Surfaces Against Rigid Materials And ArtificialArticular Cartilage,” Journal of Biomedical Material Research, 37,51-59, 1997, biodegradable anhydride prepolymers that can be crosslinked with irradiation by UV light (U.S. Pat. No. 5,902,599) andin-vivo grown articular chondrocytes in a collagen fiber or otherbio-compatible scaffold (U.S. Pat. No. 5,158,574). Other low surfaceenergy materials, such as low temperature isotropic (LTI) pyroliticcarbon, have been investigated as bearing surfaces as well.

All of these techniques are limited by one's ability to first of allfashion these materials in a conformal fashion to replicate the existingknee geometry, while at the same time, maintaining their location withinthe joint while further being able to survive the mechanical loadingconditions of the knee.

Therefore, what is needed is a uni-compartmental interpositional spacerwhich, by effectively replacing worn articular material, restores normaljoint alignment without requiring any bone resection or any means ofbone fixation and provides an anatomically correct bearing surface forthe femoral condyle to articulate against.

SUMMARY OF THE INVENTION

According to one embodiment, an implantable knee prosthesis includes abody having a substantially elliptical shape in plan and a pair ofopposed faces. A peripheral edge of variable thickness extends betweenthe faces and includes a first side, a second side opposite the firstside, a first end and a second end opposite the first end. The thicknessof the peripheral edge at the first side is greater than the thicknessof the peripheral edge at the second side, the first end and the secondend.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating an embodiment of an implantable kneeprosthesis.

FIG. 2 is a cross-sectional view taken along the line A—A of FIG. 1.

FIG. 3 is a cross-sectional view taken along line B—B of FIG. 1.

FIGS. 4 a-4 e illustrate several views of an embodiment of the device.

FIGS. 5 a-5 d illustrate several views of another embodiment of thedevice.

FIG. 6 illustrates placement of the prosthesis in a knee joint.

DETAILED DESCRIPTION

The present device is an implantable knee prosthesis in the form of aunicompartmental interpositional spacer which, by effectively replacingworn articular material, restores the normal joint alignment andprovides a congruent bearing surface for the femoral condyle toarticulate against. Further, it essentially eliminates articulationagainst the tibial surface thereby preventing further degradation of thetibial surface. Degeneration of the femoral anatomy is significantlyreduced because the conforming femoral surface of the deviceaccommodates the complex shape of the femoral condyle in extension aswell as in flexion. Insertion of the device is done via a 3 cm to 5 cmmedial parapatella incision after arthroscopic debridement of thefemoral and tibial cartilage and removal of medial meniscus toward therim along the anterior, medial and posterior portions. No bone resectionor mechanical fixation of the device is required. Only osteophytes whichinterfere with the device placement or with proper collateral ligamentalignment are removed. The device is offered in multiple thicknesses inorder to occupy the joint space and tighten the “loose suspenders”problem of the collateral ligaments. By occupying the joint space andretensioning the collateral ligaments, the unicompartmentalinterpositional spacer improves joint stability and restores the limb toa more normal mechanical alignment.

An implantable knee prosthesis 100 is illustrated in FIG. 1. Ananterior/posterior (A/P) cross-sectional view is taken along sectionline A—A and illustrated in FIG. 2. A medial/lateral (M/L)cross-sectional view is taken along section line B—B and illustrated inFIG. 3. A Coordinate System Origin (CSO) 10 is at the intersection oflines A—A and B—B. Prosthesis 100, FIGS. 1-3, includes a body 102 havinga peripheral edge 112, a first or tibial face 104 and a second orfemoral face 106.

The current mechanical structure is a compromise between the geometry ofthe femoral condyle and the kinematics of the knee. Specifically, thefemoral condyle has two major AP radii such that when the knee is fullextension, one radius position is in contact with the tibial plateauwhile, during flexion, another portion of the femoral condyle is incontact with the tibial plateau. Further complications arise when it isrecognized that the femur rotates with respect to the tibia duringflexion, thereby changing the orientation of the femoral anatomy to thetibial plateau. Much study has been dedicated to determine if anyrelationship exists in the normal human anatomy that would allow one todefine the required dimensions of the device for proper fit and functionbased on a single, easy to establish, measurable anatomic landmark.Based on a study of over 100 MRI's and 75 X-rays of human subjectsranging from 15 to 87 years of age, a relationship was establishedbetween the anteroposterior radius of the most distal portion of thefemoral condyle and the dimensions which control the geometric form ofthe device. The database revealed a range of femoral anteroposteriorradii from 32 mm to 48mm. However, it is known that the worldwide rangeis much larger because of race differences in the human anatomy.

A preferred method of construction aligns the apex of a femoral radiuswith the CSO 10. FIG. 1. The apex of a tibial surface is also generallyaligned in both the A/P and M/L directions with the CSO 10, but isseparated vertically from the CSO 10 to create the part thickness. Thesubstantially oval shape of the peripheral edge 112 is then located withrespect to the CSO 10. In general, the CSO 10 of the device is locatedat the center of the ellipse and a minor axis of the ellipse F isrelated to a major axis D by a ratio ranging from F=0.25D to 1.5D with apreferred value of=0.64D. Similar ratios can be established for all ofthe controlling dimensions of the part such that the shape in plan,i.e., as observed from above or below, femoral surface geometry, andtibial surface geometry for a normal tibial anatomy can generally bedefined by one physical A/P measurement of the patient's tibial anatomy.The appropriate thickness of the implant can be determined by measuringthe amount of joint space between the femoral and tibial surface when aminor amount of valgus (heels out, knees in) is applied to the knee.

Several aspects of the present embodiments may be referred to bymultiple terms. The term “femoral radius RA” is equivalent to bothanterior-posterior radius RA and anterior-posterior radius A.Medial-lateral radius RB may also be referred to as medial-lateralradius B. Femoral radius RC is also referred to as anterior sweep radiusC. The term “part center location ” refers to Coordinate System Origin(CSO) 10 in the drawings. Such linguistic alternatives are synonymousand interchangeable, and may be used as such in describing the presentembodiments.

Referring again to FIGS. 1-3, the preferred relationship between femoralradius RA to other joint dimensions (femoral radius is the drivingradius to all other dimensions) is as follows:

-   -   Medial-lateral radius RB=0.25A to 1.0A    -   Curve of anterior half of Femoral Radius RC=0.5A to 2.0A,        Posterior half is straight    -   Length D=0.6A to 1.4A    -   Posterior half E=0.1A to 0.75A    -   Width F=0.25A to 1.5A    -   Width from part center to medial edge G=0.096A to 0.48A    -   Anterior plan radius RH=0.16A to 0.64A    -   Posterior plan radius RM=0.16A to 0.64A    -   Radius along lateral spine area RP=0.1A to 2.0A    -   Width from part center to lateral edge Q=−0.32A to +0.32A    -   Location of transition from anterior radius to medial radius        Y=−0.32A to +0.32A (a negative value means that a dimension may        extend to an opposite side of section line A—A).

Below are the preferred ratios used to define the shape of prosthesis100 in terms of the dimension RA, i.e. the femoral radius of prosthesis100.

P=0.238A

E=0.5A

F=0.64A

H=0.32A

M=0.384A

G=0.352A

Q=0.1056A

Y=0.1152A

B=0.68A

D=RA

RC=RA

The actual shape of the present device may be tailored to theindividual. Individuals with high varus or valgus deformation due towear, degeneration, or disease, may require a device which is ofconsiderably greater thickness over the portions where wear is mostadvanced. For example, many patients who suffer from this early stage ofdegenerative arthritis will have large areas of eburnated bone along themedial edge of the tibial plateau and femoral condyle but havesignificant cartilage remaining along the tibial spine. In theseinstances the tibial surface of the implant may be thicker along themedial edge to accommodate the defects on the tibial plateau and enhancethe stability of the device. An implant made to these specificationswould be more wedge-shaped when viewed in a frontal plane, with themedial side of the implant being the larger side of the wedge. Thiswedge could be oriented in any direction to accommodate the specificlocation of significant cartilage loss for a given patient.

Alternatively, the cartilage loss can be concentrated in the centralload bearing portion of the femoral condyle. This condition results in afemoral condyle which is essentially flat when the knee is in terminalextension. In order to bridge the flattened area of the femoral condyle,the femoral surface of a specific implant size can be enlarged whilemaintaining the geometric area of the tibial surface. This modificationof the implant would prevent overhang of the tibial surface beyond theborder of the tibial plateau while providing a larger surface area todistribute the contact loads at the femoral surface. In other instances,it may be preferable to decrease the femoral surface area for a givenimplant size.

Degeneration in the medial compartment will cause the femoral condyle toshift towards the medial edge of the tibia such that the center of thefemur is no longer directly above the center of the tibia. In somepatients it may be desirable to offset the femoral surface of theimplant laterally with respect to the tibial geometry to put the femurback in a more normal alignment. Other degenerative conditions can existwhich could be accommodated by offsetting and/or rotating the femoralgeometry in a variety of directions with respect to the tibial surface.

In youthful patients, where trauma-induced damage rather than severewear or degeneration has occurred, differences in device thickness willbe more moderate. In general, the device is kidney-shaped when viewed inplan, and has a negative meniscus shape when viewed from the side, i.e.;the thickness along the periphery of the device being greater than thethickness along the center of the device. The kidney-shape in plan maybe described generally as elliptical, the shape resembling a distortedellipse.

One skilled in the art can approximate the generally elliptical shapewith a combination of straight lines and radial blends. Therefore, theterm “substantially elliptical” is intended to include all constructionmethods which yield a planar shape which is longer in one direction thanthe transverse direction, and has rounded corners.

The present invention is intended to fill in the space that results fromcartilage loss on both the femoral condyle and tibial plateau. Thethickness of the implant at the CSO should be approximately equal to thecombined amount of cartilage loss from the two boney surfaces. When animplant of proper thickness is inserted between the femur and the tibia,the limb is restored to its proper anatomic alignment and ligamentstructures around the knee are retensioned.

As previously described, the implant is thicker at the posterior edgethan at the CSO because it replicates the shape of the intact meniscus.In order for the implant to center itself on the surface of the tibia,the thick posterior edge of the device must be forced beyond the mostdistal aspect of the femur where the space between the femur and tibiais the smallest. Insertion of the implant is accomplished by forcing themedial compartment joint space open while lifting the tibia over theposterior edge of the implant. To make the insertion of the implanteasier, the implant could be separated into a femoral portion and atibial portion. The femoral portion could be positioned against thedistal femur and then the tibial portion could be inserted into the kneeseparately. The two portions could engage each other along a linear orcurved runner to insure proper orientation between the articulationsurfaces. The runner would preferably provide a slidable connection,such as a dovetail, between the two portions that would prevent themfrom separating.

For example, in the embodiments of FIGS. 4 a-4 d, an implantable kneeprosthesis is generally designated 400 and includes a body 402 having asubstantially elliptical shape in plan, and a pair of opposed faces. Afirst or tibial face 404 includes a convex surface 406. A second orfemoral face 408 includes a concave surface 410. More specifically,first face 404 and second face 408 are substantially kidney shaped.

A peripheral edge 412 of variable thickness extends between the firstface 404 and the second face 408. The peripheral edge 412 includes afirst or medial side M, a second or lateral side L, opposite the firstside M, a first or anterior end A, and a second or posterior end P,opposite the first end A.

The first side M of the peripheral edge 412 is of a first thickness T1.The second side L, the first end A, and the second end P, each have asecond thickness T2, which is less than T1. The difference between firstthickness T1 and second thickness T2 may be accomplished, for example,by providing the first face 404 with a first portion A and a secondportion B. The second portion B extends at an angle α relative to thefirst portion A. The additional thickness of T2 is thus medially shiftedon the tibial face 404 to accommodate bone loss.

Prosthesis 400 as viewed in FIG. 4 e, is similar to prosthesis 100, FIG.1, in that a dimension D, in a range of from about 0.6A to about 1.4A,is defined by the first end A and the second end P. Another dimension Fis defined by the first side M and the second side L. It has been foundthat a suitable size for body 402 is defined by the dimension F beingfrom about 0.25A to about 1.5A of the dimension D. Also a dimension Eextends from the CSO 10 of the body 402 to the first end A. It has alsobeen found that a suitable size for body 402 is also defined by thedimension E being from about 0.1 to about 0.75A of the dimension D.Further, a dimension G extends substantially from the CSO 10 of body 402to the first side M. It has further been found that a suitable size forbody 402 is further defined by the dimension G being from about 0.096Ato about 0.48A of the dimension F.

In another embodiment, FIGS. 5 a-5 d disclose an implantable kneeprosthesis generally designated 500 including a body 502 having asubstantially elliptical shape in plan, and a pair of opposed faces. Afirst or tibial face 504 includes a convex surface 506. A second orfemoral face 508 includes a concave surface 510. More specifically,first face 504 and second face 508 are substantially kidney shaped.

A peripheral edge 512 of variable thickness extends between the firstface 504 and the second face 508. The peripheral edge 512 includes afirst or medial side M, a second or lateral side L. opposite the firstside M, a first or anterior end A, and a second or posterior end P,opposite the first end A.

The first side M of the peripheral edge 512 is of a first thickness T1.The second side L, the first end A and the second end P, each havesecond thickness T2, which is less than T1. The difference between thefirst thickness T1 and the second thickness T2 may be accomplished, forexample by providing a first body piece 522 attached to a second bodypiece 524. The first body piece 522 includes the first face 504 and thesecond body piece 524 includes the second face 508. The first face 504is at an angle a relative to the second face 508.

The first body piece 522 may also have a keyed sliding interconnectionwith the second body piece 524. For example, a surface 526 of the firstbody piece 522 may include a keyway 528, and a surface 530 of the secondbody piece 524 may include a key 532 for sliding engagement with keyway528 at an interface of the respective surfaces 526 and 530. Thus, theadditional thickness T2 is medially shifted on the tibial face 504 toaccommodate bone loss.

In the embodiments of FIGS. 4 a-4 d and 5 a-5 d, the convex surface ofthe first face includes a contour angle (discussed above) which issubstantially the same as an associated contour angle of a tibialplateau. The concave surface of the second face includes a contour angle(discussed above) which is substantially the same as an associatedfemoral condyle. In this manner, the first and second faces arecontoured such that the prosthesis is self-centering between a tibialplateau and a femoral condyle as discussed above.

An exemplary use of, for example, prosthesis 400 is illustrated in FIG.6. Prosthesis 400 is positioned in a knee joint 600 between a femur 602,including the femoral condyles 604, and a tibia 606 including the tibialplateau 608. The femur 602 and tibia 606 include interconnectingcollateral ligaments 610. The device 400 illustrates the position of theposterior end P, the anterior end A, the medial side M and the lateralside L when the device 400 is inserted in the knee joint 600.

The prosthetic device of the subject invention is a unicompartmentaldevice suitable for minimally invasive, surgical implantation withoutrequiring bone resection. The device is positioned within a compartmentin which a portion of the natural meniscus is ordinarily located. Thenatural meniscus may be maintained in position or may be wholly orpartially removed, depending upon its condition. Under ordinarycircumstances, pieces of the natural meniscus which have been torn awayare removed, and damaged areas may be trimmed as necessary. In somewhatrarer instances, the entire portion of the meniscus residing in themeniscal cavity may be removed. Actually, as described hereinafter, theshape of the present device is not the same as the natural meniscus, andin most cases, will not entirely replace the meniscus.

By the term “unicompartmental” is meant that each device is suitable forimplantation into but one compartment defined by the space between afemoral condyle and its associated tibial plateau. In other words, thepresent device is not a “bicompartmental” device which, in one rigiddevice, could be inserted into both of the femoral condyle/tibialplateau compartments. In many, if not most cases, a device will beinserted into one compartment only, generally the medial compartment, asthe meniscus and associated articular surfaces in these compartments(left knee medial and right knee medial compartments) are most subjectto wear and damage. However, it is possible to insert two separatedevices into the medial and lateral compartments of the same knee, or touse two such devices that are mechanically but non-rigidly linked.

The present device is translatable but self-centering. By “translatable”is meant that during natural articulation of the knee joint, the deviceis allowed to move, or change its position. Thus, the present device isdevoid of means of physical attachment which limit its movement (forexample, screws, mating ridges and depressions, porous areas toaccommodate tissue regrowth, and the like).

The term “self-centering” means that upon translation from a firstposition to a second position during knee articulation, the device willreturn to substantially its original position as the articulation of theknee joint is reversed and the original knee position is reached. Thus,the device will not progressively “creep” toward one side of thecompartment in which it is located. Rather, the angle of attack of thefemoral condyle and/or tibial plateau bearing surfaces against thedevice will ensure that the device reversibly translates duringarticulation, maintaining the device, on average, in the same locationfor any given degree of knee articulation. The centered, rest position,of the implant is usually determined when the knee is in extension andthere is maximum contact between the femoral condyle and the device.This ability of the device to “self-center” can be compromised byinadequate tension of either one or both of the cruciate ligaments.Unbalanced or excessive cruciate ligament tension can possibly cause thedevice to locate itself in a more anterior position on the tibia, whichis less desirable.

Contrary to most devices which are composed of soft, compliant materialdesigned to assume the function of the natural meniscus which theyreplace, the present device is composed of relatively hard, relativelyhigh modulus material. Suitable materials are, for example, steel,ceramics, and reinforced and nonreinforced thermoset or thermoplasticpolymers. The device need not be made of a single material, butcomposite structures of steel/thermoplastic, steel/ceramic,ceramic/polymer, etc., may be used. Alternatively, composites of theabove materials with biologically active surfaces or components may beused. Biologically active components include surfaces that may containpharmaceutical agents to stimulate cartilage growth or retard cartilagedegeneration that may be delivered at once or in a timed-release manner.

Generally, portions of the device expected to have the most wear due toeither greater movement relative to the mating surface (i.e., relativeto the femoral condyle or tibial plateau) or high stress, may be made ofstronger, more abrasion resistant material than the remainder of thedevice when composite structures are used. This method may be ideal foruse in conjunction with cultured chondrocyte implantation (cartilagecells used as seeds) or osteochondral transplantation. Moreover, whenthe locus of damage to the articular cartilage or to a portion of thebone structure are known, the relatively constant radius of the surfaceof the present device will bridge the defective areas at these loci,thus redistributing load to healthy tissue and allowing inflamed,diseased, or other damaged areas to regenerate.

For example, a portion of the femoral condyle, tibial plateau, articularcartilage, etc., may have been damaged or may experience tissuedegeneration. The continued load experienced at such points and the wearexperienced as the knee flexes will substantially hinder theregeneration of healthy tissue. If suitable biologically activematerials, chondrocytes, etc. are applied to the damaged or degeneratedsurface to assist in tissue regeneration, these will, under ordinarycircumstances, be rapidly dissipated. If a flexible, cushiony materialis inserted within the knee compartment, the damaged area will stillexperience intimate contact with the damaged area under static loads,and will also experience continued wear and abrasion under non-staticconditions. Under such circumstances, active substances will be rapidlydissipated. However, more importantly, newly regenerated articularcartilage not having the necessary density or cohesiveness to withstandwear, will be rapidly eroded away.

The present device may be supplied with a contoured surface whichdistributes the loads evenly over regions of healthy articular cartilagewhile bridging areas where articular cartilage degeneration or damagehas occurred. Active substances may be applied at once or in atimed-release manner to the degenerated or damaged articular cartilagesurface by means of, or in conjunction with, the present device. Becausethe recess or shape of the device protects the damaged area from loadsand wear, tissue regeneration may occur without disturbance. Theregenerating tissue will have time to mature and crossline into a fullydeveloped matrix. Moreover, as regeneration proceeds, the regeneratingtissue will assume a shape dictated by the shape of the device. Growthunder these circumstances has the greatest potential for dense, orderedcartilage most closely replicating the original surface.

The hardness of the present device is preferably higher than Shore 60 D.The Shore hardness may range from that common for engineering gradeplastics to hardened steel and titanium, and preferably on the portionof the Rockwell hardness scale typical of steels, hard plastics andceramic materials. From the high hardness desired of the device, it isreadily apparent that the device functions in a manner completelydifferent from those of the prior art. The purpose of the device of thesubject invention is to achieve a span-like effect to bridge thedefective areas. However, in a composite variation, any single component(like a bioactive material component) may be softer than the supportingmaterial. Rather than deforming to distribute a load relatively equallyon the mating surfaces, the device of the present invention functions asa rigid, substantially non-deforming, self-centering bearing, which doesnot necessarily spread the load uniformly, but rather may concentratethe load upon desired points, spanning areas of imperfection. If a softand/or low modulus elastomer or thermoplastic is used for the entiredevice, not only is the load not concentrated on healthy tissue, butdamaged areas will also be subjected to loading, thereby decreasing theopportunity for the body's natural regenerative capability to function.

The high modulus of the present device thus allows for the provision ofrecessed or non-contacting areas of the device to encourage articularcartilage regeneration. In softer, lower modulus materials, thenaturally occurring loads, which may exceed 1000 lbs/in², in certaincases, will cause the softer devices to deform and allow ordinarilynon-contracting areas to contact bone or cartilage for which contact isnot desired. A flexural modulus of elasticity for load bearing portionsof the present device should therefore be preferably greater than 2×10⁵psi, and more preferably greater than 3×10⁶ psi. Portions of the devicenot exposed to the highest loads may be made of lower modulus materials,which may be softer as well (e.g., in a non-limiting sense, nylon,polyurethane, polypropylene, polyester, and the like, optionally fiberreinforced).

As indicated previously, the device of the subject invention may bemanufactured so as to substantially contain or have deposited thereon, abiologically or pharmaceutically active material. This is particularlysuitable when the device bridges a defective area of bone or articularcartilage. In such cases, the device may be provided with a coatingcontaining a biologically or pharmaceutically active material, forexample one that promotes tissue regrowth or one that decreasesinflammation. Such materials may also, and more preferably, be containedin a portion of the meniscal device. The portion may be filled withmedication, or may be filled with a gel, paste, or soft polymer materialthat releases medication over a period of time. Preferably, thismedically active portion does not actually contact, or minimallycontacts, the damaged tissue. This freedom from contact is made possibleby the surrounding bearing surfaces. Coatings may also be of a gel,paste, or polymer containing time-release medicaments. Biologically andpharmaceutically active materials are identified subsequently herein as“active materials.”

The edges of the device are rounded rather than presenting the sharpcorners of the devices of U.S. Pat. No. 5,158,574. This roundedperiphery is necessary due to the fact that the device will be allowedto move within the cavity. Movement of a device having a periphery withsharp comers would result in the potential for severe damage to thesurrounding tissue and articular surfaces, in addition to causing pain.A “depression” in the elliptical shape on the part of the device whichwill be proximate to the tibial spine will vary from patient to patient.It is possible due to the great range of variability of human anatomythat this depression might be absent in devices for some patients.However, the overall shape in plan is substantially ellipticalregardless.

The axis of rotation of the tibia on the femur is 90 degrees to the pathof the tibial plateau against the femoral condyle. The two tibialplateaus (medial and lateral) are not in the same plane with each otherbut do act in a relatively constant radius to its respective femoralcondyle. In other words, although the symmetry of the device's femoralside may be matched with the femoral condyle while the leg is in fullextension, the rotation of the tibial plateau against the femoralcondyle is along a constant axis of rotation (90 degrees to the axis ofrotation), thus the angularity of the axis of symmetry of the femoralcondyle relative to the axis of symmetry of the tibial plateau is notparallel but at some acute angle. Also, the axis of symmetry of thetibial plateau is not parallel to the path of rotation of the tibiarelative to the femur but also at some mildly acute angle. Thus, thetrue orientation of the device, regardless of the relative orientationsof symmetry of the tibial side to the femoral side is 90 degrees to thetrue axis of rotation as described in Hollister et al., “The Axes ofRotation of the Knee”, Clin. Orthopaedics and Rel. Res., 290 pp.259-268, J. B. Lippincott Co., 1993, herein incorporated by reference.Any localized positions of higher loads are self-limiting due to theability of the device to translate both rotationally and laterally whichmimics the true motion of the natural meniscus as described byHollister.

During the load bearing portion of the gait cycle, or stance phase,flexion at the knee does not typically exceed 35°. Thus, the highestcompressive loads in the knee occur with the knee substantiallyextended. The outer contours of the device are therefore designed tosubstantially mate with the corresponding tibial and femoral surfaceswhen the knee is in full extension so that the high compressive loadscan be distributed over large surface areas. The contact areas betweenthe femoral condyle and the femoral surface of the device, and thetibial plateau and the tibial surface of the device are substantiallyequivalent during extension. However, because the contour of the femoralsurface is more concave, the femoral condyle determines the position ofthe device on the surface of the tibial plateau in extension.

As the knee is flexed, the mating along the tibial surface issubstantially maintained. However, the contoured mating surfaces of thefemoral condyle and femoral surfaces of the present device can becomeincreasingly dissimilar when the joint articulates. As the knee isflexed, there is a relative external rotation and posterior translationof the femur with respect to the tibia. Thus, the contour angle of thefemur becomes more in-line with the contour angle of the tibia inflexion. This can cause relative lateral or rotational movement, in thetibial plane, between the femoral condyle and the femoral surface of thedevice. The forces generated by the increasingly different geometrycreates a rotational moment, in the tibial plane, which is resistedalong the mating tibial surfaces and which also results in a restoringforce tending to correctly locate the device along the femoral condyle.Thus, the device is self-centering to the femoral condyle, in part, as aresult of the conformity between the femoral condyle and the femoralsurface of the device.

By changing the femoral surface of the implant, it is possible to reducethe rotational moment induced during flexion by the mismatch between thefemoral surface of the implant and the femoral condyle. A preferredmethod to accommodate this motion is to have a less acute alignmentbetween the femoral and tibial axes of symmetry posterior to the A/Pmidline, thereby reducing the mismatch between the two axes in flexion.This angle is preferably 0° and can range from +/−10°. Anterior to themidline, the femoral contour is bent around a radius RC that is tangentto the posterior section of the sweep plane at the most distal point ofthe femoral A/P radius RA. This femoral surface geometry is essentiallya compromise between the different extension and flexion alignments ofthe femoral and tibial axes of symmetry.

Because the device has no physical method of attachment, the combinationof the slightly concave tibial surface and the convex femoral surfaceserves to locate the device though all ranges of motion provided thatthe collateral ligaments are in proper tension. If too thin, a devicecould be ejected from the knee compartment. By the very nature of theability to adjust for the lost articular material through the thicknessof the device, the thickness adjustment substantially eliminates theneed for a functional meniscus as a bearing surface in a severely (GradeIII or IV) degenerated knee. In these instances, the femoral surface ofthe device resides significantly above the meniscal edge, and themeniscus is completely unloaded.

The device also increases the translational stability of the knee. Theconforming shape of the femoral surface limits excessive anterior toposterior translation of the femur. As a result, this device possiblyeliminates the need for ACL reconstruction in the older patient.

Generally speaking, each knee presents a different geometry of thcrespective femoral condyles and tibial plateaus. Even with respect tothe right and left knees of a single individual, although bilateralsymmetry dictates that the left and right knee components should bemirror images, this is often only an approximation. Thus, the shape ofthe affected femoral condyle and tibial plateau (while discussed hereinin the singular, more than one pair of condyle(s)/plateau(s) may beinvolved), will have to be ascertained to determine the correct geometryof the device for a given patient.

To implant a device that possesses the characteristics required by thesubject invention, the patient's knee joint may be examined by anon-invasive imaging procedure capable of generating sufficientinformation such that one appropriately sized and shaped device may beselected. While a variety of non-invasive imaging devices may besuitable, for example X-ray devices and the like, it is preferable thatinformation as to the size and shape of the device be provided bymagnetic resonance imaging (MRI).

Two methods of non-invasive imaging for selection of a suitableprosthesis are preferred. In the first method, MRI or other non-invasiveimaging scans, optionally coupled with exterior measurements of thedimensions of the relevant tibial and femoral portions including thesurface of the particular cartilage of the tibia and femur, may be usedto establish a library of prostheses whose size and geometry differaccording to the age and size of the patient, the patient's geneticmake-up, and the like. A limited number of “standard” devices are thenmade to meet the requirements of a generic population of patients.

In this first method, a non-invasive imaging scan, such as X-ray or MRI,together with knowledge of the patient's genetic make-up, general bodytype, extent of the disease, degeneration, or trauma and the like, willenable the surgeon to select a device of the correct size and shape fromthe library for the patient. The device is then introduced byarthroscopically assisted implantation, generally limited to extensiveclean-up of existing damaged tissue, e.g., torn or particulate naturalmeniscus damage. It may also be used in conjunction with tibialosteotomy or articular surfacing procedure such as cartilagetransplantations or abrasion anthroplasty. Following insertion of thedevice, X-ray, Fluoroscopy, or MRI may be used to assess the correctpositioning of the device both introperatively as well aspostoperatively. Because the surgical procedures used are not severe,and also not irreversible, an unsuitable device may be readily removedand replaced, either with a different device from a device library, orby a custom device.

In a second method, each patient receives one or more devices that arecustom tailored for the individual by producing a contour plot of thefemoral and tibial mating surfaces and the size of the meniscal cavity.Such a contour plot may be constructed from imaging data, i.e. MRI data,by a suitable computer program. From the contour plot, the correctsurface geometry of the device is determined from the shape of therespective tibial plateau and femoral condyle and the orientationbetween the two surfaces in extension. In general, the shapes justmentioned also include the articular cartilage, which, in general, ismaintained substantially intact.

In accordance with this invention it has been discovered that the amountof varus deformity is the primary, non-invasive method for determiningthe necessary device thickness required for proper functioning of thedevice. Viewing a weight bearing anteroposterior X-ray, a cut and pasteof a line drawn through the femoral condyles and repositioned to putthem once again parallel to the tibial plateaus will yield a measurementfor the approximate device thickness.

A further understanding can be obtained by reference to the followingspecific example that is provided herein for purposes of illustrationonly and is not intended to be limiting unless otherwise specified.

For Example:

A 44-year-old male had an 18-degree flexion contracture in his rightknee. The right limb was in 5 degrees of varus alignment and the patientsuffered from significant, debilitating pain. X-rays of the affectedlimb showed significant collapse of the medial joint space as well assignificant osteophyte formation along the medial border of the femoralcondyle. Pre-operative templating of the X-ray indicated that the rightmedial condyle had a radius of approximately 46 mm. A library ofimplants was manufactured for this patient based upon the preoperativeradius measurement and the dimensional relationships established fromthe X-ray and MRI database. The library included implants with a femoralradius measuring 42 mm, 46 mm and 50 mm. Implants of 2 mm, 3 mm and 4 mmthickness were made in each size category. The patient was thenscheduled for surgery.

Arthroscopic evaluation of the joint on the day of surgery revealedgeneralized Grade III chondromalicia of the medial femoral condyle andtibial plateau with small areas of Grade IV changes. Patellofemoral andlateral joint compartment changes were mild. An arthroscopic debridementof the joint was completed and the degenerated edge of the meniscus wasresected. A small ruler was inserted through the anterior arthroscopicportal and the distance from the posterior rim to the anterior rim ofthe remaining meniscus was recorded as 42 mm. A short medianparapatellar incision was completed to expose the medial compartment ofthe knee. An osteotome and a rongure were used to remove the osteophytesalong the medial border of the femoral condyle. Plastic gagesrepresenting the different implant thicknesses were then insertedbetween the femur and the tibia to measure the amount of joint spacepresent in the medial compartment. These measurements indicated that a 4mm thick part would be required to occupy the joint space and restoretension to the medial compartment. Several trial implants were insertedinto the joint space and a fluoroscope was used to verify the fit andpositioning of each trial. This final trial reduction confirmed that theappropriate part was a 42 mm long by 4 mm thick implant. The implant wasinserted into the joint. A final check of the implant's stability andfit was performed. Careful attention was paid to the evaluation ofimplant thickness because an inappropriately thick implant could preventthe patient from achieving full extension. After all interoperativechecks were complete the incision was closed.

Postoperative X-rays revealed a 7-degree correction of the limbalignment. The implant also stabilized the knee. At 10 months offollow-up the patient is pain free and can achieve full knee extension.The patient can also achieve approximately 120 degrees of flexion.

One preferred surgical procedure which may be used to implant thisdevice can be described by the following steps:

1. Verify preoperative indications:

-   -   a. Varus determination of <5 degrees with erect AP X-ray;    -   b. Medial compartment disease only. Some lateral spurs may be        present; and    -   c. Pre-operative sizing via M/L template measurement of A/P        X-ray.

2. Standard Arthroscopy surgical prep:

-   -   a. Infiltrate knee with Lidocaine/Marcaine and Epinephine.

3. Arthroscopy:

-   -   a. Inspect lateral patello-femoral compartments for integrity,        some mild arthrosis is acceptable;    -   b. Removal of medial meniscus toward the rim along the anterior,        medial and posterior portions;    -   c. Initial arthroscopic osteophyte removal via ⅛″ osteotome and        burr to allow for valgus positioning of the knee;    -   d. Complete the removal (to the rim) of the posterior and        posterior-lateral meniscus; and    -   e. Confirm sizing of the device by measuring distance from        resected posterior meniscus to remaining anterior meniscus.

4. Medial parapatellar arthrotomy (mid-patella to tibial joint line).

5. Complete removal of visible osteophytes along the medial femoralcondyle.

6. Insert thickness gauge and size for implant thickness.

7. Insert trial component:

-   -   a. Flex knee to approximately 50+degrees to fully expose the        distal portion of the femoral condyle;    -   b. Insert trial component; and    -   c. While applying insertion pressure, apply valgus stress to the        tibia and “stretch-extend” the tibia over the trial component.

8. Check for proper sizing with “true lateral” and A/P fluoroscopeimages of the knee while in extension:

-   -   a. Ideally, the device should be within 1 mm of the A/P        boundries of the tibial plateau and superimposed over the medial        boundary.

9. Remove trial component and flush joint with saline.

10. Insert the appropriate implant.

11. Confirm proper placement and sizing with fluoroscopic images as withtrial component.

12. Maintain leg in extension and close wound after insertion of aHemovac drain.

13. Place leg in immobilizer prior to patient transfer.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

1. An implantable knee prosthesis comprising: a body having a substantially elliptical shape in plan and a pair of opposed faces, the faces including a femoral face arranged to engage a femoral surface of a knee joint and a tibial face arranged to engage a tibial surface of the knee joint, the tibial face including a first portion and a second portion, the second portion extending at an angle relative to the first portion; a peripheral edge of variable thickness extending between the faces and having a first side, a second side opposite the first side, a first end and a second end opposite the first end; and the thickness of the peripheral edge at the first side being greater than the thickness of the peripheral edge at the second side, the first end and the second end.
 2. The prosthesis as defined in claim 1 wherein a first dimension D is defined by the first end and the second end, and a second dimension F defined by the first side and the second side, where the dimension F is from about 0.25 to about 1.5 of the dimension D.
 3. The prosthesis as defined in claim 2 wherein the femoral face includes an anterior to posterior radius A and the first dimension D, where D is from about 0.6A to about 1.4A.
 4. The prosthesis as defined in claim 3 wherein the anterior to posterior radius A is equal to the first dimension D.
 5. The prosthesis as defined in claim 3 wherein the second dimension F is 0.64A.
 6. The prosthesis as defined in claim 3 wherein the femoral face includes a medial to lateral radius B of from about 0.25A to about 1.0A.
 7. The prosthesis as defined in claim 6 wherein the radius B is 0.68A.
 8. The prosthesis as defined in claim 3 wherein the femoral face includes an anterior sweep radius C to accommodate relative rotation between femur and a tibia.
 9. The prosthesis as defined in claim 8 wherein C is from about 0.5A to about 2.0A.
 10. The prosthesis as defined in claim 9 wherein C=A.
 11. The prosthesis as defined in claim 1 wherein the first side is a medial side of the prosthesis.
 12. The prosthesis as defined in claim 1 wherein the femoral face includes a concave surface.
 13. A method of providing a knee prosthesis comprising: providing a one-piece body having a substantially elliptical shape in plan and a pair of opposed faces, the faces including a femoral face arranged to engage a femoral surface of a knee joint and a tibial face arranged to engage a tibial surface of the knee joint, the tibial face including a first portion and a second portion the second portion extending at an angle relative to the first portion; extending a peripheral edge of variable thickness between the faces, the peripheral edge having a first side, a second side opposite the first side, a first end and a second end opposite the first end; and providing the thickness of the peripheral edge at the first side to be greater than the thickness of the peripheral edge at the second side, the first end and the second end.
 14. The method as defined in claim 3 further comprising: removing at least a portion of a meniscus in the knee joint.
 15. The method as defined in claim 13 further comprising: measuring for a length dimension of the prosthesis; and measuring for a thickness dimension of the prosthesis.
 16. The method as defined in claim 13 further comprising: inserting a trial component into the knee joint; verifying sizing for the trial component; and inserting the prosthesis.
 17. The method as defined in claim 13 wherein the first side includes a medial side of the prosthesis.
 18. An implantable unicompartmental knee prosthesis comprising: a unicompartmental body having a substantially elliptical shape in plan, the body having a first piece with a femoral face and a second piece with a tibial face opposing the femoral face, wherein the femoral face is arranged to engage a femoral surface of a knee joint and the tibial face is arranged to engage a tibial surface of the knee joint; a peripheral edge of variable thickness extending between the faces and having a first side, a second side opposite the first side, a first end and a second end opposite the first end; and the thickness of the peripheral edge at the first side being greater than the thickness of the peripheral edge at the second side, the first end and the second end.
 19. The prosthesis as defined in claim 18 wherein the first piece and the second piece are mutually slidably engagable and separable.
 20. The prosthesis as defined in claim 18 wherein the first and second pieces are keyed for sliding engagement.
 21. The prosthesis as defined in claim 18 wherein the femoral and tibial faces are at an angle with respect to one another.
 22. The prosthesis as defined in claim 18 wherein the first side is a medial side of the prosthesis.
 23. The prosthesis as defined in claim 18 wherein the femoral face includes a concave surface.
 24. A method of providing a unicompartmental knee prosthesis comprising: providing a unicompartmental body having a substantially elliptical shape in plan, the body having a first piece with a femoral face and a second piece with a tibial face opposing the femoral face, wherein the femoral face is arranged to engage a femoral surface of a knee joint and the tibial face is arranged to engage a tibial surface of the knee joint; extending a peripheral edge of variable thickness between the faces, the peripheral edge having a first side, a second side opposite the first side, a first end and a second end opposite the first end; and providing the thickness of the peripheral edge at the first side to be greater than the thickness of the peripheral edge at the second side, the first end and the second end.
 25. The method as defined in claim 24 wherein the first side includes a medial side of the prosthesis.
 26. The method as defined in claim 24 further comprising removing at least a portion of a meniscus in the knee joint.
 27. The method as defined in claim 24 further comprising: measuring for a length dimension of the prosthesis; and measuring for a thickness dimension of the prosthesis.
 28. The method as defined in claim 24 further comprising: inserting a trial component into the knee joint; and verifying sizing for the trial component.
 29. The method as defined in claim 24 further comprising inserting the prosthesis by positioning the first piece of the prosthesis in the knee joint and slidingly engaging the second piece of the prosthesis with the first piece of the prosthesis. 